The formation of silicon oxide on a silicon substrate is a frequently conducted process in the fabrication of semiconductor devices. One of the methods for forming silicon oxide is thermal oxidization which is carried out by subjecting a silicon wafer to an oxidizing ambient at elevated temperatures. A common objective of an oxidizing system is to obtain a high quality silicon oxide film of uniform thickness while maintaining a low thermal budget (the product of temperature and time). Methods have been developed to increase the oxidation rate and to reduce the oxidation time and temperature. Two of such methods are the dry oxidation method and the wet oxidation method by using an external torch.
The substances used to grow thermal oxides on a silicon surface are dry oxygen and water vapor. In a dry oxygen reaction, silicon oxide is formed by Si+O.sub.2.fwdarw.SiO.sub.2, while for water vapor, the reaction is Si+2H.sub.2 O.fwdarw.SiO.sub.2 +2H.sub.2. In both cases, silicon is consumed and converted into silicon dioxide.
In a dry oxidation process, silicon dioxide layers can be formed in a temperature range of 400.degree. C. .about.1150.degree. C. The process is typically performed in a resistance-heated furnace or in a rapid thermal processing chamber with heat provided by tungsten halogen lamps. In a typical dry oxidation process, a horizontal furnace tube may be used in which a batch of wafers is introduced into the furnace tube positioned in a slow moving wafer boat and then heated to an oxidation temperature in a ramp-up process. The wafers are held at the elevated temperature for a specific length of time and then brought back to a low temperature in a ramp-down process. In the dry oxidation process, oxygen mixed with an inert carrier gas such as nitrogen is passed over the wafers that are held at an elevated temperature.
A wet oxidation process can be performed by either bubbling oxygen through a high purity water bath maintained at between 85.degree. C. and 95.degree. C., or by a direct reaction of hydrogen with oxygen producing water vapor in a pyrogenic steam oxidation process.
The thermal budget required to grow a silicon oxide layer to a certain thickness is considerably smaller in a wet oxidation process than that in a dry oxidation process. The wet oxidation process for producing a silicon oxide film can therefore be carried out more efficiently and at a lower cost. However, because of a residual water content, silicon oxide films formed by the wet oxidation process exhibit a lower dielectric strength and has higher porosity to impurity penetration than silicon oxide films formed in a dry oxidation process. As a compromise, wet oxidation process is frequently used in conjunction with dry oxidation process such that a high quality oxide film can be grown with minimized oxidation time required. This is performed by beginning and ending an oxidation process in dry oxygen while using the wet oxidation process for the intermediate stage which reduces the thermal budget while increasing the overall oxide growth rate. By using this dry oxidation-wet oxidation-dry oxidation process sequence, high quality silicon oxide films can be grown on both sides of the oxide layer in order to provide properties of the three-layered film comparable to those of a single layer grown by a dry oxidation process alone.
Another benefit of the wet oxidation process is that the apparatus used for carrying out the wet oxidation may also be used to carry out a dry oxidation process. For instance, as shown in FIG. 1, a wet oxidation apparatus 10 consists of an oxidation chamber 12, an external torch 14, and a conduit 16 that connects the external torch 14 and the oxidation chamber 12 for providing fluid communication therein between. The wet oxidation apparatus 10 further includes conduit 20 for feeding an inert gas into conduit 16 for purging both the conduit 16 and the oxidation chamber 12, conduit 22 for flowing oxygen into the external torch 14 by a carrier inert gas, and conduit 24 for flowing hydrogen into the external torch 14 with an inert carrier gas. An exhaust conduit 28 takes away unused or excess water vapor in the oxidation chamber 12. The flow of gases in conduits 20, 22 and 24 is controlled by valves 30, 32 and 34, respectively.
In the wet oxidation apparatus 12, shown in FIG. 1, processing difficulties and problems frequently occur when the apparatus is used in carrying out a dry oxidation process. One of the major difficulties is residual water vapor found in the conduit 16 and in the torch 14. Residual water vapor or condensed water cumulates at the lower portion 36 of the conduit 16. The residual water vapor found in the conduit presents great difficulties in the thickness control of a thin gate oxide deposition process, causing particle issues and producing gate oxide films of low quality. Furthermore, the residual water vapor found in the conduit makes it difficult to carry out a dry oxidation process in the same oxidation chamber 12, since the residual water vapor frequently flows back into the chamber during a dry oxidation process.
It is therefore an object of the present invention to provide an apparatus for oxidizing silicon substrates that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for performing a wet oxidation process on silicon substrates that can also be advantageously used for a dry oxidation process.
It is a further object of the present invention to provide an apparatus for the wet oxidation of silicon substrates that does not have the residual water vapor problem after a wet oxidation process is conducted.
It is another further object of the present invention to provide an apparatus for the wet oxidation of silicon substrates that is equipped with an additional conduit for exhausting residual water vapor from a conduit connecting an external torch and an oxidation chamber.
It is still another object of the present invention to provide an apparatus for the wet oxidation of silicon substrates that does not have the residual water vapor problem in the conduit such that the apparatus can be subsequently used for a dry oxidation process.
It is yet another object of the present invention to provide a method for conducting a wet oxidation process on silicon substrates in an oxidation chamber and then immediately conducting a dry oxidation process in the same chamber without the residual water vapor problem.
It is still another further object of the present invention to provide a method for preventing vapor contamination in an oxidation chamber by providing the oxidation chamber with an additional conduit for exhausting any residual water vapor in a conduit that connects an external torch to the oxidation chamber.
It is yet another further object of the present invention to provide an apparatus for conducting a wet oxidation process on silicon substrates by providing an additional conduit for exhausting any residual water vapor left in the conduit or the chamber such that the apparatus can be immediately used for conducting a dry oxidation process.